Evaporation is a thin-film deposition process utilized for creating thin-film layers of gold onto a wafer. FIG. 1 shows a representative example of an evaporative process 100 for depositing thin-film layers of gold. The tool utilized in the process 100 is commonly referred to as a planetary evaporation tool 101. Solidified gold material 102 (refer to hereafter as a “gold evaporative source”) is placed in a crucible 103. Electron beam or resistance heating elements can serve as the heat source 104 to melt the gold evaporative source 102. The melting of the gold evaporative source 102 creates a liquid pool 105 as shown in FIG. 1. The gold evaporative source in molten form is further heated until vaporization occurs. The gold vapor traverses upwards within the interior of the tool 101, and ultimately condenses and deposits as a thin-film onto one or more surfaces of a series of wafers 106, each of which is shown situated on a wafer holder 107. In this manner, one or more layers of the gold from evaporation source 102 may be deposited on the wafers 106.
The evaporative process 100 occurs under vacuum conditions and within a prescribed range of operating parameters. Operating parameters such as power input, crucible liner, heat transfer and deposition rate may be monitored and adjusted if necessary to ensure they remain within target set point ranges. There are instances during the evaporative process 100 when the deposition rate may gradually ramp down or sharply decrease below its target set point range. In order to compensate for the reduced deposition rate, the power of the tool is typically increased. However, oftentimes, the increased power imparted to the tool 101 and heat sources 104 can substantially alter the boiling characteristics of the gold evaporation source 102 such that it attains a vigorous and high-energy state. The boiling at the increased power levels can be associated with a rapidly moving or turbulent molten liquid pool, particularly at the free surface. Depending upon the severity of the molten pool turbulence, molten gold liquid can be inadvertently ejected from the crucible towards the wafers 106 in the form of liquid droplets 108, as shown in FIG. 1. This phenomenon is known as “spitting” and is recognized as detrimental to film quality by virtue of the creation of in-film defects known as nodules or particles contained or embedded within the deposited gold film. Such nodules are further problematic as they can significantly lower production yields and lead to device malfunction by a breakdown of the layered structures of the device.
The phenomenon for spitting is not entirely understood, but is believed to be attributed, at least in part, to the conditions and parameters of the process 100, which if left uncontrolled, has the potential to influence the likelihood of spitting. In this regard, conventional means for overcoming spitting has involved adjusting several parameters, including controlling power input to the tool 101; controlling heat transfer to the solidified gold material 102; and selection of a suitable liner material or the crucible 103. Although such process conditions and parameters can be adjusted, spitting remains a prevalent problem during the gold evaporative process 100. Additionally, end-users are often limited in their degree of freedom to alter conditions and parameters from established set points or target values. Generally speaking, in the microelectronics industry, because the source tools have previously been qualified to operate at established processing parameters which have demonstrated the ability to precisely and reliably produce acceptable wafers with the required thin-film layers of deposited gold, the processing parameters preferably are to remain unchanged when performing the evaporative process 100.
As an alternative or in addition to varying process conditions, end-users have utilized a gold evaporation source having a purity of at least 99.999% to prevent spitting. However, the problem of inadvertent liquid droplet ejection from the crucible still occurs.
In view of the on-going challenges, there remains an unmet need for a gold evaporative process that overcomes spitting and is capable of producing gold films without in-film defects.